1. Field of This Invention
This invention relates to a pipette system for the volumetric withdrawal and linear deposition of a suspended material, in particular cells or particles. This invention also relates to a pipette system consisting of a capillary containing a piston inserted in such a way as to prevent the passage of liquids.
2. Prior Art
Suspensions are routinely produced and prepared for medical and diagnostic purposes, e.g., examinations of blood or cells. A standard method is the plane preparation of a cell suspension on a microscope slide, a second slide being used to spread the cell suspension over the whole surface of the slide or to obtain a thin layer. The disadvantage of such process is that the cells and the liquid adhere to the auxiliary slide that is used to distribute the suspension. This means that part of the suspension is wasted, and it is no longer possible to count the exact number of cells in the original volume. In addition, the distribution of the cells is plane and sometimes uneven. If greater magnification is used, making the area to be examined greater than the field of view of the microscope, marginal cells make automated analysis of the cells difficult, and automated evaluation of the sample becomes complicated and expensive.
Devices are known which permit linear application of the sample to the carrier and automatic evaluation of the material under examination (e.g., German O.S. 1,900,808). In such case the suspension is deposited linearly on the carrier which is in the form of a ribbon, the width of the deposit corresponding to the diameter of the field of view of the microscope. Deposition is effected by a drawing pen formed by two metal tongues (the distance between the tongues may be varied) and the suspension volume to be deposited passes between them. The tip of the metal tongues must come into contact with the carrier during deposition in order to permit the suspension to flow out. With such a device it is not possible to achieve a volumetric, proportional deposition of particles from suspensions in the form of a line of specific breadth and length within a given space of time and at a given deposition speed because both particles and liquid adhere to the surfaces of the metal tongues that come into contact with the suspension and because the rate of discharge of the suspension is dependent both on its surface tension and on the wetting properties of the carrier. A further major disadvantage is that it is necessary to clean the system prior to each change of sample in order to prevent the entrainment of cells.
Even when the metal tongue discharge device is replaced by a micropipette system where a piston is inserted into a capillary tube so as to prevent the passage of liquid and in such a way that it can be moved axially by means of a miniature electric motor with reduction gear towards the orifice of the capillary according to the volume to be taken up and later discharged and so that it seals the capillary on completion of the process, the demands, especially for larger suspension volumes (e.g., &gt;1 .mu.l) with a pre-selectable width of smear, are still not satisfied.
The width of the cell smear that can be achieved corresponds to the outside diameter of the tip of the pipette and is only slightly wider than the diameter of the capillary if the tube has a conically tapered shape at the orifice. The particle suspension is discharged by force by the motion of the piston and when a motor is used the rate of discharge is infinitely variable. The piston seal ensures that the capillary is completely emptied, leaving no residues. An important advantage of this type of pipette is that the forced discharge of the suspension from the pipette eliminates the need for the tip of the pipette to come into contact with the carrier, thus preventing wear of the tip and damage to the surface of the carrier. Material is no longer carried over into the next sample, and the pipette does not need to be cleaned so frequently. This system can be used, for example, for the contactless deposition of a smear 0.3 mm wide and between 5 and 50 cm long from 0.15 .mu.l of liquid, where the system has a capillary with an inside diameter of 0.2 mm, the outside diameter at the tip being 0.3 mm, and where the piston stroke is ca. 4.8 mm. The disadvantage of this kind of pipette is, however, that it can only be used for volumes in the range of 0.1 to 0.5 .mu.l. For example, in order to deal with a volume greater than 1.5 .mu.l the piston stroke would have to be more than 50 mm, but this would cause substantial instability, making the equipment useless.
In the field of medical engineering, and in particular for early diagnosis of cancer, it must be possible to treat cell suspension volume of up to several .mu.l with the above accuracy. It is simple to withdraw a larger quantity of cell suspension by increasing the diameter of the capillary. But it is then no longer possible to achieve the required width of the smear (0.3 mm or less) since the width of the smear is directly dependent on the diameter of the tip of the capillary and cannot be smaller than the diameter of the capillary.